Lacunar cyclidenes

Ni(III) complexes often exhibit equilibrium with Ni(II) ligand radical species. For example, the Ni(III) complex of lacunar cyclidene is octahedral [NimL(CH3CN)2]3+ at low temperature, but at high temperature it transforms into the Ni(II) complex with an oxidized ligand radical [NinL+]3+, identifiable by the absorption at 590 nm. The Ni(III) complexes and Ni(II) species with an oxidized ligand radical exhibit a thermal equilibrium (Eq. 12) (105). 

As mentioned earlier, cyclidenes are cyclic multidentate imino-donor ligands. The lacunar cyclidenes developed by Busch and co-workers... 

A picnic-basket porphyrin is reminiscent of the lacunar cyclidenes mentioned earlier, in which an alkyl chain tied the ends of the cyclidene together and formed a pocket for the complexation of dioxygen. Cobalt complexes of the picnic-basket porphyrins 31 bind dioxygen reversibly at room temperature with high oxygen affinities (84). The oxygen affinity increases as the basket size decreases (as determined by the length... 

The lacunar cyclidenes are synthesized from a precursor tetraaza macrocycle containing a pendant vinyl carbon attached to a methoxy group, which readily undergoes addition-elimination reactions with diamines to give the vaulted bridge bicycle.36... 

The synthetic routes may often involve template directed condensations, a widely used reaction being the (carbonyl + amine) to imine condensation that efficiently leads to a variety of Schiff-base macrocycles [2.58-2.60, A.7, A.14], macrobicyclic cryptands [2.61-2.63] and lacunar cyclidene ligands [2.60, 2.64]. 

REVERSIBLE COBALT(II) AND IRON(II) DIOXYGEN CARRIERS OF TOTALLY SYNTHETIC LACUNAR CYCLIDENE COMPLEXES ... 

The lacunar cyclidene ligands are the first family of totally synthetic ligands to form both iron(II) and cobalt(II) Oj carriers. The molecular structure of the lacunar cyclidene complexes is most simply represented by the flat projection (la), while the stereochemistry is shown in Ib. Cyclidene refers to the parent macrocycle encircling the metal ion and the lacuna is the permanent void created by bridging the cleft arising from the saddle shape of the cyclidene macrocycle. 

The starting material for both of these target molecules is the neutral nickel(II) complex [3,11-diacetyl-4,10-dimethyl-1,5,9,13-tetraazacyclohexa-deca-1, 3, 9, ll-tetraenato(2 —)-K N ]nickel(II), ([Ni(Ac2Me2[16]-tetra-enatoN )]). This complex is prepared by the procedure described earlier. The lacunar cyclidene ligands are constructed on the nickel(II) ion, and the formation of the second macrocyclic ring is a template reaction. The first two reactions in the Scheme are required unless R is to be methyl. Thus, the synthesis of the cobalt complex does not require these steps, whereas... 

Figure 1. Structures of cydidene complexes 1) lacunar cyclidene comply 2) vaulted cyclidene...

The syntheses of two complexes are described here the cobalt(II) lacunar cyclidene complex in which R =—(CH2)e— and = R = —CH3, and the iron(II) complex in which R =m-xylylene, R = —CH2C6H5, and R = —CgHs- The former is the most studied example of a lacunar cyclidene oxygen carrier, and the latter represents the first nonporphyrin iron complex capable of forming a stable dioxygen adduct at room temperature. 

Fig. 8. Cavity sizes and the acconunodation for CO in lacunar cyclidene complexes (distances reported in angstrom units).

Table I. Equilibrium constants for CO binding to iron(II) lacunar cyclidene complexes in acetonitrile at 0°C.

---